1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming control method, and in particular to an image forming apparatus and an image forming control method that carry out full-color image formation by primarily transferring a toner image formed on a photosensitive drum onto an intermediate transfer member and secondarily transferring the toner image on the intermediate transfer member onto a recording medium.
2. Description of the Related Art
Conventionally, there has been known an image forming apparatus that forms a full-color image by forming a latent image on a photosensitive drum according to an electrophotographic method (laser beam method) and developing the latent image by causing toners of respective colors to adhere to the latent image, then primarily transferring the toner images on the photosensitive drum onto an intermediate transfer member and secondarily transferring the toner images on the intermediate transfer member onto a recording medium. An image forming apparatus of this type employs a technique which forms, in carrying out image formation on a recording medium such as thick paper or an OHP sheet, a full-color image by writing toner images of the respective colors (by exposing the photosensitive drum) starting from a reference position on an image carrier (i.e., the photosensitive drum and the intermediate transfer medium) to thereby form the toner images on the image carrier. Japanese Laid-Open Patent Publication (Kokai) No. 05-216323 discloses a technique that, to obtain a sharp image in HP mode or lossy mode the processing speed (i.e. rotational speed of the photosensitive drum) is reduced to 1/n without changing the scanning speed of an optical writing means so that optical writing is carried out for only one scanning line out of every n scanning lines, that is, a technique that reduces the processing speed during image formation and carries out image formation for lines that are reduced in number by an amount corresponding to the drop in speed in a subscanning direction during exposure of the photosensitive drum, transfers toner images onto a recording medium, and fixes the toner images.
This technique that carries out image formation for a reduced number of lines can be easily implemented when the reduced processing speed is ½ or ¼ of the normal processing speed, but when the reduced processing speed is ⅓ or ⅔ of the of the normal processing speed, there has been the problem that it is necessary to use complicated hardware circuits of a laser exposure device and the like that carries out exposure processing. To solve this problem, there has been already developed a method that carries out an image forming process for forming images on an image carrier without changing the processing speed but changes the processing speed for carrying out processes including transferring toner images onto a recording medium and subsequent processes (for example, Japanese Laid-Open Patent Publication (Kokai) No. 07-140845).
However, the above prior art has the following problem. That is, when image formation is carried out by the above conventional image forming apparatus on plain paper or a like recording medium without changing the processing speed, in the case where a marking or the like that is formed in advance on an image carrier (intermediate transfer member) is detected and the detected position is used as a reference position (home position) during image writing, there is the problem that image writing cannot be started before the home position is detected. As one solution, it can be envisaged that the image carrier is stopped at a suitable position for subsequent image formation after completion of post-processing (processing such as cleaning off remaining toner from the image carrier) that follows the completion of image formation.
However, when the image carrier (intermediate transfer member) is a belt-shaped member, the image carrier is stretched over a plurality of rollers and rotatively driven, which leads to deterioration of the material of the image carrier due to tension. To avoid such deterioration, it is not possible to stop the image carrier exactly at the same position. Since it is thus not possible to always stop the image carrier at a suitable position following the post-processing mentioned above, time is required to detect the home position, depending on the position of the home position at the start of image formation, and the image formation can be only commenced after waiting for the time required for up to one full rotation of the image carrier at the maximum. This results in that an FCOT (First Copy Out Time) that is a period of time taken from the start of image formation (a process from charging to fixing with exposure, developing, and transferring in between) to discharging of a first recording medium for which image formation has been completed is excessively long.